Until recently, power tongs were the established method used to run casing or tubing strings into or out of petroleum wells, in coordination with the drilling rig hoisting system. This power tong method allows such tubular strings, comprised of pipe segments or joints with mating threaded ends, to be relatively efficiently assembled by screwing together the mated threaded ends (make-up) to form threaded connections between sequential pipe segments as they are added to the string being installed in the well bore; or conversely removed and disassembled (break-out). But this power tong method does not simultaneously support other beneficial functions such as rotating, pushing or fluid filling, after a pipe segment is added to or removed from the string, and while the string is being lowered or raised in the well bore. Running tubulars with tongs also typically requires personnel deployment in relatively higher hazard locations such as on the rig floor or more significantly, above the rig floor, on the so called ‘stabbing boards’. The advent of drilling rigs equipped with top drives has enabled a new method of running tubulars, and in particular casing, where the top drive is equipped with a so called ‘top drive tubular running tool’ or ‘top drive tubular running tool’ to grip and perhaps seal between the proximal pipe segment and top drive quill. (It should be understood here that the term top drive quill is generally meant to include such drive string components as may be attached thereto, the distal end thereof effectively acting as an extension of the quill.) Various devices to generally accomplish this purpose of ‘top drive casing running’ have therefore been developed. Using these devices in coordination with the top drive allows rotating, pushing and filling of the casing string with drilling fluid while running, thus removing the limitations associated with power tongs. Simultaneously, automation of the gripping mechanism combined with the inherent advantages of the top drive reduces the level of human involvement required with power tong running processes and thus improves safety.
In addition, to handle and run casing with such top drive tubular running tools, the string weight must be transferred from the top drive to a support device when the proximal or active pipe segments are being added or removed from the otherwise assembled string. This function is typically provided by an ‘annular wedge grip’ axial load activated gripping device that uses ‘slips’ or jaws placed in a hollow ‘slip bowl’ through which the casing is run, where the slip bowl has a frusto-conical bore with downward decreasing diameter and is supported in or on the rig floor. The slips then acting as annular wedges between the pipe segment at the proximal end of the string and the frusto-conical interior surface of the slip bowl, tractionally grip the pipe but slide or slip downward and thus radially inward on the interior surface of the slip bowl as string weight is transferred to the grip. The radial force between the slips and pipe body is thus axial load self-activated or ‘self-energized’, i.e., considering tractional capacity the dependent and string weight the independent variable, a positive feedback loop exists where the independent variable of string weight is positively fed back to control radial grip force which monotonically acts to control tractional capacity or resistance to sliding, the dependent variable. Similarly, make-up and break-out torque applied to the active pipe segment must also be reacted out of the proximal end of the assembled string. This function is typically provided by tongs which have grips that engage the proximal pipe segment and an arm attached by a link such as a chain or cable to the rig structure to prevent rotation and thereby react torque not otherwise reacted by the slips in the slip bowl. The grip force of such tongs is similarly typically self-activated or ‘self-energized’ by positive feed back from applied torque load.
In general terms, an embodiment of the “Gripping Tool” of WIPO Patent Application PCT/CA2006/000710 may be summarized as a gripping tool which includes a body assembly, having a load adaptor coupled for axial load transfer to the remainder of the body, or more briefly the main body, the load adaptor adapted to be structurally connected to one of a drive head or reaction frame, a gripping assembly carried by the main body and having a grip surface, which gripping assembly is provided with activating means to move from a retracted position to an engaged position to radially tractionally engage the grip surface with either an interior surface or exterior surface of a tubular work piece in response to relative axial movement or stroke of the main body in at least one direction, relative to the grip surface. A linkage is provided acting between the body assembly and the gripping assembly which, upon relative rotation in at least one direction of the load adaptor relative to the grip surface, results in relative axial displacement of the main body with respect to the gripping assembly to move the gripping assembly from the retracted to the engaged position in accordance with the action of the activating means.
This gripping tool thus utilizes a mechanically activated grip mechanism that generates its gripping force in response to axial load or stroke activation of the grip assembly, which activation occurs either together with or independently from, externally applied axial load and externally applied torsion load, in the form of applied right or left hand torque, which loads are carried across the tool from the load adaptor of the body assembly to the grip surface of the gripping assembly, in tractional engagement with the tubular work piece.
The grip surface of prior art gripping tools are generally comprised of a coarse profiled and hardened surface typical of tong dies known to the art, where such dies are designed to be sufficiently “sharp” so as to provide a consistent and reliable tractional engagement with the work piece for a gripping tool's grip ratio. Where grip ratio is defined as the normal force (radial load for tubulars) acting between the grip surface and the work piece divided by the magnitude of the shear force (arising from applied hoisting and torsional loads) and by definition must exceed the inverse of the effective coefficient of friction existing between the grip surface and the work piece to prevent slippage. “Sharper” dies, with less contact area, generally penetrate the work piece at lower normal forces providing a higher effective friction coefficient at the correlative lower hoisting load than “duller” dies but this has the side effect of causing greater indentation depth at greater loads leaving localized regions of plastic deformation on the surface of the work piece which are undesirable in certain applications.
As grip surfaces wear the die tooth tips become more rounded and the tooth tip area increases such that the effective coefficient of friction tends to decrease (at the same normal stress). In addition, work pieces with hardened, inconsistent, or coated surfaces offer reduced coefficient and require a tool with a higher grip ratio or a more aggressive grip surface to safely run. Similarly a higher grip ratio is typically required at lower magnitudes of normal force. The present invention is directed to this need.